Gauging by the number of sorties flown, and thus about the number of aircraft available to the…

Before the US strike on Shayrat AB, the SyAAF had some 3–4 operational MiG-21s at Hama; about 10 operational MiG-23ML/MLDs and MiG-23BNs at…

Gauging by the number of sorties flown, and thus about the number of aircraft available to the Assad-Regime on average: yes, that’s quite accurate.

Before the US strike on Shayrat AB, the SyAAF had some 3–4 operational MiG-21s at Hama; about 10 operational MiG-23ML/MLDs and MiG-23BNs at Hama, Shayrat and Dmeyr; MiG-25s were out of service; MiG-29s hardly ever flown; but there were up to 16–18 Su-22s at Shayrat, Tiyas and Dmeyr. Up to 20 L-39s — of which about a dozen was operational — rounded up the picture; with remark that these are slow and can carry only a friction of weapons load carried by other types.

These 50–60 aircraft were flying between 45 and 60 sorties a day before 7 April.

Ever since, they’re flying 20 sorties (max 30, like yesterday, when L-39s are flying a lot).

Furthermore, clear evidence (photos and videos) is meanwhile available for destruction of at least 9 MiG-23MLDs and Su-22s at Shayrat.

Thus, I would go as far as to say: yes, 20% of available aircraft were destroyed. Even more so: destruction of these 20% aircraft resulted in 40% decrease in number of sorties flown.